“I saw an idea once where the solar plant pumps water into a reservoir during the day and produces electricity at night with hydro.”
I’ve heard that one, too. But where is all that water coming from and where is it going to be stored?
Depends a lot on geography. In the upstate region of the Carolinas, they use excess juice from their nukes to pump water up into reservoirs in the Appalachian foothills; during the day they release that water to spin turbines to help handle peak demand.
As regards solar and wind, I seriously doubt they’re economical enough to have any more than limited value in isolated areas - they can augment traditional methods, but in most cases any major solar or wind operation will need traditional back-up (nowadays gas-fired turbines, I believe): the grid doesn’t tolerate interruptions real well, and any sudden occurrence of overcast or calm on a segment heavy reliant on solar and/or wind is an invitation for a major grid crash.